140 PICTURESQUE SKETCHES 



contained the mouth and the eyes ; and is, in fact, 

 the head, containing what may be regarded as a 

 true brain. The rest of the animal is immediately 

 contiguous, and includes the stomach and other or- 

 gans ; and all the soft parts are inclosed in what 

 is called a mantle or sac, a thick muscular skin, 

 which keeps them together in their places and 

 enables them to act in concert, but offers no defence 

 against an external enemy. 



The animal of the nautilus, thus constructed, hav- 

 ing considerable powers of locomotion and a complex 

 organization, was inclosed in a stony habitation some- 

 what resembling many common univalve shells. But 

 such an appendage adds considerably to the specific 

 gravity of the whole mass which, even without it, could 

 hardly be lighter than water ; and with it, unless by 

 some special contrivance, would for ever remain at the 

 bottom of the sea. The contrivance is a very sim- 

 ple one, and consists merely in a provision by which, 

 as the animal grows in size, it from time to time 

 builds off a cup-shaped wall upon the soft rounded 

 surface of the hinder part of the body, leaving as 

 it goes a space behind it, which is occupied only by 

 air or some gaseous substance, and acts as a float. 



Proceeding in this way, and building a succession 

 of these walls, there is ultimately formed what is 

 called a chambered shell ; and all the chambers but 

 the outer one, (that containing the animal,) being filled 

 with air, are in a condition to support the body 

 and shell in the water without sinking. A commu- 

 nication is kept up between the sac inclosing the heart 

 (the pericardium) and the various chambers by a tube 

 passing through all the walls in succession ; and it 



